The prior art discloses that trapping techniques can gently immobilize and manipulate small objects, which is useful in many applications. In particular, optical trapping, which uses the momentum change of light scattering to impart forces on small objects, has been applied to trap dielectric nanospheres, carbon nanotubes, semiconductor nanowires, and metal nanoparticles. Optical trapping has great potential in microbiology applications because of its ability to trap tiny bio-particles without inducing damage. Direct optical trapping of biological particles, however, has been limited to relatively large objects, for example, living cells, bacteria, 300 nm long tobacco mosaic virus particles and DNA strains. For smaller particles, a common approach is to tether the end of the particle onto a large micrometer-sized bead. This introduces steric hindrance, hydrodynamic effects and experimental complexity (e.g. the need for binding).
Direct trapping and manipulation of smaller biological and other particles such as nanoparticles, quantum dots, and colloidal particles remains challenging, primarily because the difficulty of trapping a nonresonant dielectric particle dramatically increases as the particle size decreases. For example, using a traditional perturbative trap, the laser power needed to trap a particle, for a given average time, scales with the inverse fourth power of the particle. Due to the above mentioned disadvantages it is desired to directly manipulate nanoparticles with optical forces without tethering, and improved methods and apparatus are needed.